Know Your Enemy:
IDing remote hosts, without them knowing
Last Modified: 24 May 2000
One of the challenges of network
security is learning about the bad guys. To understand your threats
and better protect against them, you have to Know
Your Enemy. Passive Fingerprinting is a method to learn more
about the enemy, without them knowing it. Specifically, you can determine
the operating system and other characteristics of the remote host using
nothing more then sniffer traces. Though not 100% accurate, you can
get surprisingly good results. Craig Smith has developed a
proof of concept tool based on the concepts covered in this paper.
Also, the subterrain crew has developed
siphon, a passive port mapping and OS fingerprinting tool.
Traditionally, Operating System
fingerprinting has been done using active tools, such as queso or nmap.
These tools operate on the principle that every operating system's IP stack
has its own idiosyncrasies. Specifically, each operating system responds
differently to a variety of malformed packets. All one has to do
is build a database on how different operating systems respond to different
packets. Then, to determine the operating system of a remote host,
send it a variety of malformed packets, determine how it responds, then
compare these responses to a database. Fyodor's nmap
is tool of choice when using this methodology. He has also written
detailed paper on this.
Passive fingerprinting follows the
same concept, but is implemented differently. Passive fingerprinting
is based on sniffer traces from the remote system. Instead of actively
querying the remote system, all you need to do is capture packets sent
from the remote system. Based on the sniffer traces of these packets,
you can determine the operating system of the remote host. Just like
in active fingerprinting, passive fingerprinting is based on the principle
that every operating system's IP stack has its own idiosyncrasies.
By analyzing sniffer traces and identifying these differences, you
may be able determine the operating system of the remote host.
There are four areas that we will look at to determine the operating system (however
there are other signatures that can be used). These signatures are:
By analyzing these factors of a packet, you
may be able to determine the remote operating system. This system
is not 100% accurate, and works better for some operating systems then
others. No single signature can reliably determine the remote
operating system. However, by looking at several signatures and combining the
information, you increase the accuracy of identifying the remote host.
An example would be the easiest way to explain. Below is
the sniffer trace of a system sending a packet.
This system launched a mountd exploit against me, so I want to learn more
about it. I do not want to finger or nmap the box, that could give
me away. Rather, I want to study the information passively.
This signature was captured using snort,
my sniffer of choice.
TTL - What the operating system sets
the Time To Live on the outbound packet
Window Size - What the operating system
sets the Window Size at.
DF - Does the operating system set the
Don't Fragment bit.
TOS - Does the operating system set
the Type of Service, and if so, at what
TCP TTL:45 TOS:0x0 ID:56257
***F**A* Seq: 0x9DD90553
Ack: 0xE3C65D7 Win: 0x7D78
Based on our 4 criteria, we identify
We then compare this information to
a database of signatures. First, we look
at the TTL used by the remote host. From our sniffer trace above,
you can see the TTL is set at 45. This most likely means
it went through 19 hops to get to us, so the original TTL was set
at 64. Based on this TTL, it appears this packet was sent from a Linux or
FreeBSD box, (however, more system signatures need to be added to the database).
This TTL is confirmed by doing a traceroute to the remote host.
If you are concerend about the remote host detecting your traceroute,
you can set your traceroute time-to-live (default 30 hops), to be one
or two hops less then the remote host (-m option). For example, in this
case we would do a traceroute to the remote host, but using only 18
hops (traceroute -m 18). This gives you the path information
(including their upstream provider) without actually touching the remote host.
For more information on TTLs, check out this
Research Paper on Default TTL values.
Window Size: 0x7D78 (or 32120
DF: The Don't Fragment bit is set
The next step is too compare the Window size. I have found the Window Size to
be another effective tool, specifically what Window Size is used and how often the size
changes. In the above signature, we see it set at 0x7D78, a default Window
Size commonly used by Linux. Also, Linux, FreeBSD, and Solaris tend to maintain
the same Window Size throughout a session (as this one did). However, Cisco
routers (at least my 2514) and Microsoft Windows/NT Window Sizes are constantly changing.
I have found that Window Size is more accurate if measured after the initial three-way handshake
(due to TCP slow start). For more information on Window Size, see Stevens,
"TCP/IP Illustrated, Volume 1" Chapter 20.
Most systems use the DF bit set, so this is of limited value. However,
this does make it easier to identify the few systems that do not use the DF flag
(such as SCO or OpenBSD). After
further testing, I feel that TOS is also of limited value. This seems
to be more session based then operating system. In other words, its not
so much the operating system that determines the TOS, but the protocol
used. TOS defintely requires some more testing. So,
based on the information above, specifcally TTL and Window size,
you can compare the results to the database of
signatures and with a degree of confidence determine the OS (in
our case, Linux kernel 2.2.x).
Keep in mind, just as with Active Fingerprinting, Passive Fingerprinting
has some limitations. First, applications that build their own packets
(such as nmap, hunt, teardrop, etc) will not use the same signatures as
the operating system. Second, it is relatively simple for a remote host
to adjust the TTL, Window Size, DF, or TOS setting on packets. For example,
to change the default TTL value:
Solaris: ndd -set /dev/ip ip_def_ttl 'number'
Linux: echo 'number' > /proc/sys/net/ipv4/ip_default_ttl
However, by combining a variety of different packets and signatures, in this case TTL
and Window Size, you can reliably approximate the remote system.
Other Signatures and Uses
We are not limited to the four signatures discussed so far. There are
other areas that can be tracked, such as initial sequence numbers,
IP Identification numbers, TCP or IP options. For example, Cisco routers
tend to start IP Identification numbers at 0, instead of
randomly assigning them. Also, ICMP payloads can be used.
Max Vision discusses using ICMP payload type or TCP options for remote host
identification. For example, Microsoft ICMP REQUEST payloads contain the
alphabet, while Solaris or Linux ICMP REQUEST payloads have number and
symbols. For TCP Options, the option Selective Acknowledgement
is commonly used by Windows and Linux, but not commonly used by
FreeBSD or Solaris. With Maximum Segment Size (MSS), most operating
systems use a MSS of 1460, however Novell commonly uses 1368, and some
FreeBSD variants may use a MSS of 512. Another source of signatures is packet state, what
type of packet is being used. To quote Fyodor "For example, the initial SYN request
can be a gold mine (as can the reply to it). RST packets also have some
interesting features that can be used for identification." These and other
signatures can be combined with the signatures listed above to help identify
remote operating systems.
Passive fingerprinting can be used for several other purposes. It can be used
by the bad guys as 'stealthy' fingerprinting. For example, to determine
the Operating System of a 'potential victim', such as a webserver, one only
needs to request a webpage from the server, then analyze the sniffer traces.
This bypasses the need for using an active tool that can be detected by various
IDS systems. Also, Passive Fingerprinting may be used to identify remote
proxy firewalls. Since proxy firewalls rebuild connection for clients, it may
be possible to ID the proxy firewalls based on the signatures we have discussed.
Organizations can use Passive Fingerprinting to identify 'rogue' systems on
their network. These would be systems that are not authorized on the network.
For example, a Microsoft or Sun shop can quickly identify 'rogue' Linux
or FreeBSD systems that mysteriously appeared on their network. Passive
Fingerprinting can be used to quickly inventory an organizations operating systems without
touching or imapcting any systems or network performance. For individuals conducting security assessment, Passive Fingerprinting also allows one to quickly identify critical systems (such as Unisys Mainframe).
Building the Database
was built by testing a variety of systems with the Telnet, FTP, HTTP, and SSH protocol.
More testing needs to be conducted using various other protocols, sessions, and systems.
If you have any signatures to add to the database, please send them to
email@example.com. I am especially interested in TCP or IP options or systems
not listed in the database.
Passive fingerprinting gives you the ability to learn about the enemy,
without them knowing it. Though no single piece of information can positively
identify a operating system, by combining several signatures, you can make an
approximation of the remote system. A big thanks to the following people for their
help and ideas:
Subterrain Siphon Project